Charged particle beam apparatuses have many functions in a plurality of industrial fields, including, but not limited to, inspection of semiconductor devices during manufacturing, exposure systems for lithography, detecting devices and testing systems. Thus, there is a high demand for inspecting specimens within the micrometer and nanometer scale.
Micrometer and nanometer scale process control, inspection or structuring, is often done with charged particle beams, e.g. electron beams or ion beams, which are generated and focused in charged particle beam devices, such as Scanning Electron Microscopes (SEM) or Focused Ion Beam (FIB) tools. Charged particle beams offer superior spatial resolution compared to e.g. photon beams, due to their short wavelengths.
A prominent tool for inspections is the Scanning Electron Microscope (SEM), an example of which is shown in FIG. 1. The SEM 1 uses a primary electron beam 7 as a means to probe the surface structure of a given specimen 3. An interaction of the primary electron beam 7 with the specimen 3 causes secondary electrons 17 to be released into a backward direction with respect to the primary electron beam 7 where they are detected by an electron detector 15. By scanning the primary electron beam across the specimen 3 and determining the rate of the released secondary electrons at each scan position, an image of the surface of the specimen 3 with high spatial resolution is obtained. The spatial resolution of the image is essentially given by the size of the beam focus.
While a SEM uses a focused primary electron beam to image a specimen, a FIB instead uses a focused primary ion beam, typically gallium ions. During scanning of the primary ion beam over the specimen, secondary electrons and ions are generated which may be collected to form an image of the surface of the specimen. The FIB can also be incorporated in a system with both electron and ion beam columns, allowing the same feature to be investigated using either of the beams.
For detection and classification of topographic defects, like particles, at the surface of specimens, a good topographic contrast is necessary. In scanning beam applications, topographic contrast may be obtained by detection of secondary electrons or ions having different starting angles from the specimen. In scanning beam tools the secondary electrons or ions produced at the specimen are usually collected over a broad range of starting angles for imaging the specimen. When using for instance a low energy SEM utilizing a retarding field objective lens for imaging a specimen, substantially all secondary electrons produced at the surface of the specimen may be attracted inside the objective lens and may therefore be detected.